The limitations of aperture sights became very apparent to me this July when I helped organize and competed in a Run 'n Gun event. I was going to run it with my scoped AR, but didn't get a chance to get it zeroed and ran it with my iron sighted M1 instead. I was one of the last to run, and ended up finishing up almost at dusk. One of the stages required hitting an 18-20" plate at 400 yards. I had already ran about 2 miles when I hit that stage. The plate was painted red. It was sitting in red dirt, in the shade, with the sun setting behind it, and it was starting to get dark. I couldn't see the plate through that aperture to save my life. I was having to pop my head up to try to see it, then come back down on my sights and guess where it was and work that area over. I had 1:30 to get 3 hits, and used up all my time without getting one. The next course of fire involved advancing downrange and getting 2 hits from each of 4 positions, from 300 to 200 yards, in 5 minutes. I STILL couldn't see the target at 300 yards, and used up most of my time there. I didn't end up completing that course of fire in time either.
I know if I had my scope, I would have been able to see the target no problem. Hell, I probably would have done better with notch sights. I almost certainly would have done better with an Aimpoint.
It isn't my vision... I have 20/15. And a man-sized target at 400 yards is no problem for me with that rifle when I can easily see the target... but life isn't always that accommodating. That's why I have optics on all my serious use rifles.